Beginning users often assume that elevated polylines must be created as 3dPolys. If the elevations of a polyline vary, that is the case. However, if the elevations of a polyline are consistent (such as a proposed contour) a standard (lightweight) polyline is best.

If you are creating them with the PLINE command you can assign the elevation with the first pick. In response to the start point enter .XY and press enter (or space), AutoCAD will let you pick the point then asks for the elevation. All remaining picks will be on that elevation.